I look at the ratio between material $'s and labor $'s (cost). Example: if misc materials+fixtures+gear is $100,000 and labor dollars are $50,000, that is a 2/1 ratio. If I mark this up 20% for OH&P, that would be a $180,000 price. If this job goes south, and my labor over-run is 50%, I would lose $20,000. This is not likley with a good GC, but it happens. This is why I shy away from jobs where the owner furnishes fixtures, gear, etc. It becomes an all-labor job, and is too risky. Or, I will mark the job up as-if I was furnishing these materials. I will also not be competitive, so why bother bidding?
Most "good" electrical jobs run around 2.8/1 and up (3/1, 4/1). The above example, I would have to mark it up around 30% to cover my risk. I probably won't get that job.